#1062 and #1068 causes perf degradation

We put requests in a buffer, and pull the buffer async to process them. The buffer is building up easily after pulling in #1062 and #1068.

The issue may be caused by the fact that these two diffs would make write serialized.

Start by reviewing the changes referenced in #1062 and #1068, then trace how requests enter the buffer and are pulled asynchronously for processing. Investigate whether writes became serialized and measure whether that causes the buffer to build up. Done means the performance degradation and buffer growth are resolved.

Written by the indexing model from the issue text.